Measurement of caspase activity in individual cells reveals differences inthe kinetics of caspase activation between cells

Most forms of apoptosis involve activation of caspases and it is likely tha t differences between cells in their ability to activate caspases contribut es to the responsiveness of any given cell within a population to apoptotic stimuli. To study the molecular mechanisms that underlie such differences, it is necessary to measure caspase activity in individual cells. Here, we describe a method that allows the continuous monitoring of caspase activity in individual, living mammalian cells. This approach allows studies of the kinetics of caspase activation to be performed in individual cells within a population, We demonstrate that in a group of cells where some cells die and some cells survive in response to the same stimulus, the cells that die can be differentiated from those that survive based on the amount of caspa se activity in each cell several hours before death occurs.